Trainspotters will relish Trevithick's Tunnel, site of the first test of Richard Trevithick's steam-powered locomotive. In 1804 it was the first in the world to haul a load on rails – it lugged 10 tonnes of iron for 9.5 miles, at a speed of 4mph. It's off the A470 in Pentrebach, 1.25 miles south of Merthyr.
